This invention relates generally to the field of conventional cylinder type door locks, and more particularly to an improved plug element of permutation type employing a plurality of combination tumblers adapted to replace an existing pin tumbler plug element.
Pin tumbler constructions employ a key having notches along the shank portion thereof which correspond to the heights of cylinder plug pins in a given lock. When inserted into the plug through an exposed opening, the proper key aligns all of the parallel pin tumblers in the lock such that the parting planes of each tumbler are in congruent relation with the surface of the plug permitting the plug to be turned to release a locking structure disposed therebehind. If an incorrect key is inserted, a difference of as little as one sixty fourths of an inch in any tumbler prevents the plug from turning. However, because of the presence of an opening in the exposed surface of the plug for the insertion of the key, the lock may be mechanically picked using a technique known as "torking" in which each tumbler is individually overcome in serial fashion by the insertion of an elongated picking tool.
Combination or mutation locks are well known in the lock art, and have been extensively used in padlocks and safe door constructions. While they are not totally immune to picking operations, considerably greater skills are necessary to overcome the combination tumblers which are not physically accessible to a potential lock-pick. At the present state of the art, there is no conveniently available lock of permutation type suitable for use as a substitute for conventional lock sets.